Fabrics and veins in the forearc: A record of cyclic fluid row at depths of <15 KM

Fluid flow through much of the forearc is channeled along faults and fractures, so the distribution and textural history of veins and deformation fabrics can be used to evaluate forearc plumbing. From shallow to deeper levels, fabric elements depict a network of fluid conduits that fluctuate between dilatancy and collapse. In general, there is a transition from particulate flow in soft sediments deformed on the upper slope or near the toe of the prism to grain scale diffusive mass transfer associated with metamorphism of low porosity rocks in the interior of the forearc. With increasing depth of burial, veins vary from zones of distributed grain boundary failure ("mud-filled veins") to cracks that follow grain boundaries and fill with dirty carbonate to cracks that break across grains and fill with clean calcite or quartz. Deformation fabrics vary with depth from semipervasive mud-filled veins, kink bands, shear bands, microfaults, and scaly foliation to slaty cleavage. Fault zones display a scaly fabric that records dissolution or local collapse of a more open grain network. Dilatancy within these zones is inferred based on observations of carbonate and quartz veins and the requirement that the scaly fabrics act as fluid conduits capable of maintaining observed geochemical and thermal anomalies. Much of the fluid expelled from the downgoing slab may migrate upward within the scaly fabric at the top of the underthrust sediment pile to be ultimately vented near the toe of the prism. Observations of vein distributions within the more metamorphosed wedge interior, however, indicate diffuse movement of fluid along systems of hydrofractures that develop in regional zones of low permeability. Differences in hydrogeology between some active convergent margins reflect the range of incoming sediment packages as well as the episodic nature of fluid flow. Fluid flow may be cyclic, in which case the frequency of events may depend on the transient distribution of open fractures, the rate at which excess fluid pressures develop within fracture arrays, the rate at which fractures seal, the strain rate, and the nature of the relationship between the seismic events and fluid flow events.